linux-kernel - [tip: x86/urgent] x86/bugs: Don't fill RSB on VMEXIT with eIBRS+retpoline

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Message-ID: <174419906403.31282.6211937821580800684.tip-bot2@tip-bot2>
Date: Wed, 09 Apr 2025 11:44:24 -0000
From: "tip-bot2 for Josh Poimboeuf" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 Josh Poimboeuf <jpoimboe@...nel.org>, Ingo Molnar <mingo@...nel.org>,
 Amit Shah <amit.shah@....com>, Nikolay Borisov <nik.borisov@...e.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Vitaly Kuznetsov <vkuznets@...hat.com>,
 Sean Christopherson <seanjc@...gle.com>,
 David Woodhouse <dwmw2@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject:
 [tip: x86/urgent] x86/bugs: Don't fill RSB on VMEXIT with eIBRS+retpoline

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     18bae0dfec15b24ec14ca17dc18603372f5f254f
Gitweb:        https://git.kernel.org/tip/18bae0dfec15b24ec14ca17dc18603372f5f254f
Author:        Josh Poimboeuf <jpoimboe@...nel.org>
AuthorDate:    Tue, 08 Apr 2025 14:47:33 -07:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Wed, 09 Apr 2025 12:41:55 +02:00

x86/bugs: Don't fill RSB on VMEXIT with eIBRS+retpoline

eIBRS protects against guest->host RSB underflow/poisoning attacks.
Adding retpoline to the mix doesn't change that.  Retpoline has a
balanced CALL/RET anyway.

So the current full RSB filling on VMEXIT with eIBRS+retpoline is
overkill.  Disable it or do the VMEXIT_LITE mitigation if needed.

Suggested-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Reviewed-by: Amit Shah <amit.shah@....com>
Reviewed-by: Nikolay Borisov <nik.borisov@...e.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: Sean Christopherson <seanjc@...gle.com>
Cc: David Woodhouse <dwmw2@...radead.org>
Link: https://lore.kernel.org/r/84a1226e5c9e2698eae1b5ade861f1b8bf3677dc.1744148254.git.jpoimboe@kernel.org
---
 arch/x86/kernel/cpu/bugs.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 9926509..a10b37b 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1617,20 +1617,20 @@ static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_
 	case SPECTRE_V2_NONE:
 		return;
 
-	case SPECTRE_V2_EIBRS_LFENCE:
 	case SPECTRE_V2_EIBRS:
+	case SPECTRE_V2_EIBRS_LFENCE:
+	case SPECTRE_V2_EIBRS_RETPOLINE:
 		if (boot_cpu_has_bug(X86_BUG_EIBRS_PBRSB)) {
-			setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT_LITE);
 			pr_info("Spectre v2 / PBRSB-eIBRS: Retire a single CALL on VMEXIT\n");
+			setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT_LITE);
 		}
 		return;
 
-	case SPECTRE_V2_EIBRS_RETPOLINE:
 	case SPECTRE_V2_RETPOLINE:
 	case SPECTRE_V2_LFENCE:
 	case SPECTRE_V2_IBRS:
-		setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT);
 		pr_info("Spectre v2 / SpectreRSB : Filling RSB on VMEXIT\n");
+		setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT);
 		return;
 	}